1. Technical Field of the Invention
The present invention relates generally to a spark plug with increased durability for internal combustion engines which may be used in automotive vehicles, co-generation systems, or gas feed pumps.
2. Background Art
Internal combustion engines used in, for example, automotive vehicles employ spark plugs to ignite an air-fuel mixture. Such spark plugs are being increasingly required to improve combustion in the engine for reducing the fuel consumption or emissions thereof. The improvement of the combustion requires improvement of thermal efficiency and compression ratio in the engine which may, however, result in self-ignition of the mixture in combustion chambers of the engine, thus leading to engine knocking.
The engine knocking may be eliminated by improving a cooling system such as a water jacket provided around the spark plugs to enhance the cooling of the engine.
Modern engines are quipped with a direct injection mechanism or a variable valve-timing mechanism for enhancing the performance thereof and complex in structure of an engine head.
The improvement of cooling of the engine, therefore, requires a decrease in installation space for the spark plug. For instance, Japanese Utility Model First Publication No. 5-55489 teaches techniques for decreasing the diameter of a plug installation thread formed on a metal shell of the spark plug. This, however, results in need for decreasing the diameter of a porcelain insulator of the spark plug, which gives rise to a decrease in mechanical strength thereof. This increases the possibility of breakage of the porcelain insulator due to engine vibrations.
There are known ignition devices having an ignition coil installed just above the spark plug in direct connection therewith for the purpose of minimizing electrical noises arising from the distributor or simplifying a high-voltage wiring arrangement. The ignition coil is fitted on the porcelain insulator of the spark plug and fixed on a head cover installed on the engine head. The spark plug is installed directly on the engine head.
The above structure, however, has the problem in that engine vibrations may be transmitted to a joint between the ignition coil and the spark plug, so that an undesirable mechanical load or stress is applied to the head of the porcelain insulator of the spark plug. Specifically, when the engine vibrations applied to the engine head in which the spark plug is installed and the head cover in which the ignition coil is installed differ in direction or phase from each other, it will cause a bending force or stress to act on the porcelain insulator. Thinning of the porcelain insulator, therefore, increases the possibility of breakage of the porcelain insulator due to the engine vibrations.
Electrical insulation of the spark plug from the ignition coil working to produce high voltage is usually achieved by fitting a coil boot of the ignition coil with the porcelain insulator firmly. Specifically, the high voltage, as developed by the ignition coil, is transmitted to the center electrode of the spark plug through a terminal of the spark plug and a conductor installed in the porcelain insulator to produce sparks in a spark gap between the center electrode and the ground electrode to ignite the mixture in the engine. When the coil boot is loosen from the head of the porcelain insulator, so that a greater gap exits between them, it may result in the so-called flashover in which the current leaks to the head of the porcelain insulator, which gives rise to a lack of spark in the spark gap, thus decreasing the ignitability of the mixture in the engine.
Particularly, increasing in the spark gap caused by wear of the electrodes of the spark plug will result in a rise in voltage of discharge. Further, deterioration or hardening of the coil boot caused by long use of the spark plug results in a decrease in electrical insulation ability thereof, which may result in a discharge to the surface of the porcelain insulator (i.e., the flashover), thereby leading to misfiring in the engine.
Conversely, the coil boot is fitted on the head of the porcelain insulator too tightly, a greater effort is required to pull the coil boot out of the spark plug. Particularly, when the coil boot is fit on the spark plug, air within the coil boot will expand during running of the engine and contract during stop of the engine. The removal of the spark plug is usually made when the engine is at rest and cool. A negative pressure or vacuum is, thus, developed in the coil boot, thus resulting in an increase in effort required to pull the coil boot from the spark plug. If the coil boot is pulled by force, it may result in damage to the coil boot or breakage of or cracks in the porcelain insulator. Specifically, firm fitting of the coil boot with the head of the porcelain insulator facilitates ensuring of electrical insulation therebetween, but may be a factor of damage to the coil boot or breakage of or cracks in the porcelain insulator.